Method of producing a magnetic recording medium and a magnetic recording medium formed thereby

ABSTRACT

A perpendicular magnetic recording medium has a magnetic recording layer with ferromagnetic crystalline grains and nonmagnetic and nonmetallic grain boundary region surrounding the grains. The surface of its underlayer, before forming the magnetic recording layer, is exposed to an O 2  or N 2  atmosphere or an atmosphere of rare gas and O 2  or N 2 , to attach the O 2  or N 2  as nucleation sites for promoting growth of the nonmagnetic and nonmetallic region. By forming the magnetic recording layer thereafter, both ferromagnetic crystalline grains and the nonmagnetic and nonmetallic grain boundary region are formed from the initial stage of the growth of the magnetic recording layer. Thus, a magnetic recording layer having excellent segregation structure can be formed.

This is a continuation of application Ser. No. 10/390,332, filed Mar.17, 2003.

BACKGROUND

A perpendicular magnetic recording system with recording magnetizationperpendicular to the medium surface has been contemplated as analternative to a conventional longitudinal magnetic recording system, toattain higher recording density. A perpendicular magnetic recordingmedium is principally composed of a magnetic recording layer of a hardmagnetic material, an underlayer for aligning the magnetic recordinglayer to an aimed direction, a protective layer for protecting thesurface of the magnetic recording layer, and an underlayer of a softmagnetic material having a function to converge a magnetic flux that isgenerated by a magnetic head for recording in the magnetic layer.

The soft magnetic underlayer can be omitted since recording is possiblewithout it, although it can improve media performance. A medium withoutthe soft magnetic underlayer is called a single-layered perpendicularmagnetic recording medium, and a medium having the soft magneticunderlayer is called a double-layered perpendicular magnetic recordingmedium. A perpendicular magnetic recording medium, as well as alongitudinal magnetic recording medium, must perform high thermalstability compatible with low media noise in order to achieve highrecording density.

In conventional longitudinal magnetic recording media, variouscompositions and structures of a magnetic recording layer and materialsfor a nonmagnetic underlayer have been proposed. Practical magneticrecording layers use an alloy of Co and Cr (hereinafter referred to asCoCr) and obtain magnetically isolated magnetic grains by segregatingthe chromium at the grain boundary. Another type of a magnetic recordinglayer, called a granular magnetic recording layer that uses nonmagneticand nonmetallic substance, such as oxide or nitride, has been proposed.

In a magnetic recording layer of the CoCr, the substrate must be heatedto a temperature higher than 200° C. during the deposition of the layerto sufficiently segregate the chromium. On the other hand, the granularmagnetic recording layer has a feature where the nonmagnetic andnonmetallic substance segregates even if the substrate heating isomitted. The magnetic recording layer of CoCr and the granular magneticrecording layer can be applied to a perpendicular magnetic recordingmedium as well, establishing perpendicular anisotropy by controllingcrystal alignment in the recording layer with the aid of an underlayer,for example.

In a perpendicular magnetic recording medium, however, it is equallydifficult to segrate chromium in the magnetic recording layer using CoCras in a longitudinal magnetic recording medium. On the hand, aperpendicular magnetic recording layer employing a granular magneticlayer makes chromium separation easier than in the CoCr recording layer.As a result, magnetic interaction between the grains can be suppressed,leading to low media noise. However, the granular magnetic recordinglayer in a thin film thickness of about 10 nm or less does not givesufficient segregation structure, resulting in poor isolation betweengrains, and causing media noise.

Because recording in a perpendicular magnetic recording medium isideally done with a sharp perpendicular magnetic field induced by amagnetic recording head, it is desirable to form the magnetic recordinglayer as thin as possible. If an initial growth layer with suchinsufficient segregation is formed, it is difficult to obtain a usefulthin magnetic recording layer. Consequently, lower noise and higherrecording density have not been attained with granular magneticrecording layers.

Accordingly, there is a need to develop a perpendicular magneticrecording medium that exhibits low noise and high recording density byachieving excellent segregation structure in the magnetic layer. Thepresent invention addresses this need.

SUMMARY OF THE INVENTION

The present invention relates to a method of producing a perpendicularmagnetic recording medium and to a perpendicular magnetic recordingmedium produced thereby. Such a perpendicular magnetic recording mediumis suitable for mounting on a variety of magnetic recording devices.

One aspect of the present invention is a method of producing aperpendicular magnetic recording medium comprising the steps ofsequentially laminating an underlayer, a magnetic recording layer, aprotective layer, and a liquid lubricant layer on a nonmagneticsubstrate. The magnetic recording layer comprises ferromagneticcrystalline grains and nonmagnetic grain boundary region comprisedmainly of oxide or nitride surrounding the crystalline grains. Thelaminated underlayer is exposed to an atmosphere containing O₂ or N₂before laminating the magnetic recording layer. The atmosphere cancontain rare gas.

The underlayer can be composed of Ru or an alloy of Ru, which can beRuW, RuCu, RuC, RuB, or RuCoCr. A seed layer can be laminated beforelaminating the underlayer. The seed layer can be composed of a Ni-basealloy, which can be NiFe, NiFeNb, NiFeB, NiFeSi, or NiFeCr. If a seedlayer is used, a soft magnetic underlayer can be laminated before theseed layer. The soft magnetic underlayer can be composed of crystallinematerials of a NiFe alloy and a Sendust alloy (FeSiAl), fine crystallinematerials of FeTaC and CoTaZr, or an amorphous cobalt alloy of CoZrNb.

Another aspect of the present invention is a perpendicular magneticrecording medium produced by the method above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating an example ofembodiment of a perpendicular magnetic recording medium according to thepresent invention.

FIG. 2 is a chart showing dependence of magnetic cluster size on themagnetic recording layer thickness obtained from MFM evaluation forExamples 1 and 2 and Comparative Examples 1 and 2.

FIG. 3 is a chart showing dependence of standard deviation of magneticcluster size on the magnetic recording layer thickness obtained from MFMevaluation for Examples 1 and 2 and Comparative Examples 1 and 2.

FIG. 4 is a chart showing dependence of the normalized media noise inthe case of the magnetic recording layer thickness of 15 nm on linearrecording density obtained from the evaluation of magnetic parametricperformance for Examples 1 and 2 and Comparative Examples 1 and 2.

DETAILED DESCRIPTION

Both the magnetic portion and the nonmagnetic and nonmetallic grainboundary region can be simultaneously formed from the initial stage ofthe recording layer formation, and the ferromagnetic crystalline grainscan be magnetically separated. Specifically, an oxide or a nitride canform a nonmagnetic and nonmetallic underlayer. The underlayer is exposedto an atmosphere containing O₂ or N₂ before depositing the granularmagnetic recording layer onto the underlayer. The O₂ or N₂ attached onthe substrate surface with the underlayer can act as nucleation sites togrow the nonmagnetic and nonmetallic region causing separation offerromagnetic crystalline grains from the initial growth layer of themagnetic recording layer.

Some aspects of preferred embodiments of the present invention will bedescribed with reference to the accompanied drawings in the following.FIG. 1 is a schematic cross-sectional view illustrating an example ofembodiment of a perpendicular magnetic recording medium according to thepresent invention. The perpendicular magnetic recording medium has astructure comprising at least an underlayer 2, a magnetic layer 3, and aprotective layer 4 sequentially formed on a nonmagnetic substrate 1. Aliquid lubricant layer 5 is further formed on the layers.

The nonmagnetic substrate 1 can be composed of a NiP-plated aluminumalloy, strengthened glass, or crystallized glass, which are currentlybeing used in a conventional magnetic recording medium. A plasticsubstrate made of a resin, such as polycarbonate, polyolefin, or thelike, also can be used when a temperature of substrate heating is heldunder about 100° C.

The underlayer 2 is preferably composed of a metal with a hexagonalclosest packed (hcp) structure or an alloy of such a metal, or a metalwith a face centered cubic (fcc) structure or an alloy of such a metal.The metal with the hcp structure includes Ti, Zr, Ru, Zn, Tc, and Re.The metal with the fcc structure includes Cu, Rh, Pd, Ag, Ir, Pt, Au,Ni, and Co. Of the materials exemplified above, Ru or Ru alloy exhibitsexcellent effect because of weak reactivity when exposed to O₂ or N₂.Although a thin film is desirable, a thickness at least 3 nm ispreferable, which thickness of crystal growth provides acceptable massor structure.

A seed layer 12 can be provided beneath the underlayer 2 to improvealignment of the underlayer 2. Although the seed layer can benonmagnetic, a material with soft magnetic property is preferable foracting as a part of a soft magnetic underlayer in a structure of adouble-layered perpendicular magnetic recording medium. Examples of thematerial of the seed layer 12 exhibiting soft magnetic property includenickel-base alloys such as NiFe, NiFeNb, NiFeB, NiFeSi, and NiFeCr.

If a seed layer 12 is provided under the underlayer 2, a soft magneticunderlayer 11 can be further provided under the seed layer forconverging the magnetic field generated by a magnetic head to constructa double-layered perpendicular magnetic recording medium. Materials thatcan be used for the soft magnetic underlayer 11 include crystallinematerials of a NiFe alloy and a Sendust alloy (FeSiAl), fine crystallinematerials of FeTaC and CoTaZr, and an amorphous cobalt alloy of CoZrNb.

While the optimum thickness of the soft magnetic underlayer 11 dependson the structure and characteristic of a magnetic head used forrecording, a thickness from 10 nm to 500 nm is desirable considering abalance with productivity.

The magnetic recording layer 3 has a structure composed of ferromagneticcrystalline grains and a nonmagnetic grain boundary region surroundingthe grains. The magnetic recording layer employs a granular magneticrecording layer, in which the magnetic grain boundary region is composedof nonmagnetic nonmetallic substance. The ferromagnetic crystallinegrains are preferably composed of a CoPt alloy, an FePt alloy, or one ofthese alloys that contains additive element(s) selected from Cr, Ni, Nb,Ta, and B.

The nonmagnetic metallic substance of the nonmagnetic grain boundaryregion is preferably an oxide or a nitride, for example, an oxide or anitride of an element selected from Cr, Co, Si, Al, Ti, Ta, Hf, Zr, Y,and Ce. The ferromagnetic crystalline grains need to exhibit magneticanisotropy perpendicular to the film surface in order to be used in aperpendicular magnetic recording medium.

The substrate surface before forming the magnetic recording layer 3,namely the surface of the underlayer 2, is exposed to an atmosphere ofO₂ or N₂, or to an atmosphere containing O₂ or N₂. The atmosphere caninclude rare gas. The O₂ or N₂ is attached as nucleation sites forgrowing the nonmagnetic nonmetallic region. After that, the magneticrecording layer 3 is formed. As a result, both the ferromagneticcrystalline grains and the grain boundary region of the nonmagneticnonmetallic substance are formed from the initial stage of the magneticlayer formation. Thus, a magnetic recording layer having excellentsegregation structure can be formed.

The protective layer 4 can be composed of a thin film of mainly carbon.The liquid lubricant layer 5 can be formed of perfluoropolyetherlubricant.

The following describes specific examples of embodiments ofperpendicular magnetic recording media according to the presentinvention.

In the first Example, the nonmagnetic substrate used was a chemicallystrengthened glass substrate with smooth surface (N-5 glass substrate,manufactured by HOYA Corporation). After cleaning, the substrate wasintroduced into a sputtering apparatus. A soft magnetic underlayer ofCoZrNb 300 nm thick was formed using a target of Co-5Zr9Nb under argongas pressure of 5 mTorr. Subsequently, a seed layer of NiFeNb 20 nmthick was deposited using a soft magnetic Ni-base alloy target ofNi-12Fe9Nb under argon gas pressure of 5 mTorr.

Then, an underlayer of Ru 20 nm thick was formed using a Ru target underargon gas pressure of 30 mTorr. After that, the resulting substrate wasexposed to an argon gas atmosphere containing 2% of O₂ for 10 sec. Thepressure of the gas mixture of argon and oxygen was 5 mTorr, and theflow rate was 60 sccm.

Then, a magnetic recording layer of CoCrPt—SiO₂ was deposited using atarget of 91(Co-10Cr17Pt)-9SiO₂ under argon gas pressure of 30 mTorr.The thickness of the magnetic recording layer was varied in the range of10 to 30 nm. After depositing a carbon protective film 8 nm thick usinga carbon target, the resulting substrate was taken out from the vacuumchamber.

Finally, a liquid lubricant layer 2 nm thick was formed ofperfluoropolyether by means of a dip-coating method. Thus, adouble-layered perpendicular magnetic recording medium was produced.Deposition of the magnetic recording layer was conducted by means of anRF magnetron sputtering method; deposition of all other layers wasconducted by means of a DC magnetron sputtering method.

In the second Example, a double-layered perpendicular magnetic recordingmedium was produced in the same manner as in Example 1, except that theunderlayer and magnetic recording layer were formed with a RuWunderlayer 15 nm thick, exposing to an atmosphere of argon containing 3%of N₂ for 10 sec, and a magnetic recording layer of CoCrPt—SiN using a92(Co-10Cr15Pt)-8SiN target.

As a first Comparative Example, a double-layered perpendicular magneticrecording medium was produced in the same manner as in Example 1, exceptthat the Ar+O₂ atmosphere exposure before forming the CoCrPt—SiO₂magnetic recording layer was omitted.

As a second Comparative Example, a double-layered perpendicular magneticrecording medium was produced in the same manner as in Example 2, exceptthat the Ar+N₂ atmosphere exposure before forming the CoCrPt—SiNmagnetic recording layer was omitted.

Results of magnetic property evaluation for the Examples and ComparativeExamples are described below. The magnetic performance was measured bymagnetic Kerr effect. Table 1 shows coercivity Hc for the magneticrecording layers having a thickness of 15 nm. Specifically, Table 1gives the coercivity Hc for the magnetic recording layers havingthickness of 15 nm obtained from magnetic property evaluation onExamples 1 and 2 and Comparative Examples 1 and 2. TABLE 1 Hc [Oe]Example 1 4,320 Example 2 3,460 Comp Example 1 4,000 Comp Example 23,200Thickness of magnetic recording layer: 15 nm

Squareness ratio S for every Examples and Comparative Examples was 1.0.Comparing Example 1 with Comparative Example 1, the Example 1, whichexperienced the exposure to the Ar+O₂ atmosphere, demonstratedimprovement in Hc as compared to the Comparative Example 1, which didnot experience the exposure. Similarly, comparing Example 2 withComparative Example 2, the Example 2, which experienced the exposure tothe Ar+N₂ atmosphere, showed improvement in Hc. Thus, the exposure tothe atmosphere containing O₂ or N₂ promoted segregation structure andcontributed to improvement in Hc.

FIG. 2 and FIG. 3 show dependences of a diameter d [nm] and a standarddeviation of the diameter σ [nm], respectively, of a magnetic clustersize on the thickness of the magnetic recording layer for Examples andComparative Examples. The values of magnetic cluster size were obtainedby MFM measurement for each of the AC-demagnetized media. It isgenerally apparent that both Examples 1 and 2, which were subjected tothe exposure, significantly decreased both d and σ as compared toComparative Examples 1 and 2, which did not experience the exposure.

Focusing on the case of 10 nm thickness of the magnetic recording layerin particular, in Comparative Examples 1 and 2, both the diameter d andthe standard deviation σ are substantially larger in comparison, whichindicates that separation of the ferromagnetic crystalline grains wasinsufficient in the initial growth layer, resulting in the large valuesof the magnetic cluster sizes and the deviations thereof.

Examples 1 and 2, which were subjected to the exposure, in contrast,both the diameter d and the standard deviation σ are small even at thethickness of 10 nm of the magnetic recording layer, which indicates thatO₂ or N₂ acted as nucleation sites for the growth of the nonmagneticnonmetallic region and promoted separation of the ferromagneticcrystalline grains from the initial stage of the growth of the magneticrecording layer.

FIG. 4 shows dependence of normalized media noises on linear recordingdensity, obtained from measurement of magnetic parametric performancefor 15 nm thickness of the magnetic recording layer in the Examples andthe Comparative Examples. The magnetic parametric performance wasobtained by the measurement using a spin-stand tester equipped with aGMR head. As is apparent from FIG. 4, the media noise significantlydecreased in Examples 1 and 2, which were subjected to the exposure, ascompared to Comparative Examples 1 and 2, which did not experience theexposure. Considering in combination with the above-described evaluationresults on the magnetic cluster size, the noise reduction has beenattained by virtue of sufficient separation of the magnetic crystallinegrains from the initial stage of the growth of the magnetic recordinglayer, which in turn resulted from the exposure.

Table 2 shows SNR values at 400 kFCI and 600 KFCI for the magneticrecording layer thickness of 15 nm, obtained from evaluation on themagnetic parametric performance for Examples 1 and 2 and ComparativeExamples 1 and 2. TABLE 2 SNR[dB] at 400 [kFCI] SNR[dB] at 600 [kFCI]Example 1 16.6 4.32 Example 2 12.4 1.86 Comp Example 1 10.9 0.69 CompExample 2 9.5 0.33Thickness of magnetic recording layer: 15 nm

The SNRs were obtained from the similar evaluation of the magneticparametric performance to in the case of the above-described normalizedmedia noise. Reflecting the high Hc and low noise described above, theSNR in the Examples 1 and 2, which were subjected to the exposure,demonstrated substantial improvement as compared to Comparative Examples1 and 2, which did not experience the exposure.

By introducing nucleation sites on the surface on which a granularmagnetic recording layer is to be formed, for the purpose of promotinggrowth of a nonmagnetic and nonmetallic grain boundary phase, theferromagnetic crystalline grains can be separated from the initialgrowth layer of the magnetic layer. The effect results from theconstitution of the invention in which a granular magnetic recordinglayer is used comprising nonmagnetic grain boundary region ofnonmagnetic and nonmetallic oxide or nitride, and exposing the substratesurface with an underlayer before forming the magnetic layer to anatmosphere containing O₂ or N₂ to introduce nucleation sites for thegrowth of the nonmagnetic and nonmetallic region. Accordingly, magneticinteraction between the ferromagnetic crystalline grains decreases toreduce media noise, and at the same time, to attain a thin magneticlayer. Therefore, high recording density can be accomplished in aperpendicular magnetic recording medium according to the presentinvention.

Given the disclosure of the present invention, one versed in the artwould appreciate that there may be other embodiments and modificationswithin the scope and spirit of the present invention. Accordingly, allmodifications and equivalents attainable by one versed in the art fromthe present disclosure within the scope and spirit of the presentinvention are to be included as further embodiments of the presentinvention. The scope of the present invention accordingly is to bedefined as set forth in the appended claims.

The disclosure of the priority application, JP PA 2002-077024, in itsentirety, including the drawings, claims, and the specification thereof,is incorporated herein by reference.

1. A method of producing a perpendicular magnetic recording mediumcomprising the steps of: forming an underlayer over a nonmagneticsubstrate: exposing the underlayer in an atmosphere containing ° 2 or N₂after the underlayer has been fully formed: and then forming a magneticrecording layer on the exposed underlayer, wherein the magneticrecording layer comprises ferromagnetic crystalline grains andnonmagnetic grain boundary region composed mainly of oxide or nitridesurrounding the crystalline grains.
 2. A method of producing aperpendicular magnetic recording medium according to claim 1, whereinthe magnetic recording layer is deposited using a target composed of anoxide or a nitride.
 3. A method of producing a perpendicular magneticrecording medium according to claim 1, wherein the magnetic recordinglayer is deposited using a target composed of an oxide or a nitride ofan element selected from Cr, Co, Si, Al, Ti, Ta, Hf, Zr, Y, and Ce.
 4. Amethod of producing a perpendicular magnetic recording medium accordingto claim 1, wherein the ferromagnetic crystalline grains are composed ofa CoPt alloy, an FePt alloy, or one of these alloys that containsadditive element selected from Cr, Ni, Nb, Ta, and B, the nonmagneticgrain boundary region is composed of an oxide or a nitride of an elementselected from Cr, Co, Si, Al, Ti, Ta, Hf, Zr, Y, and Ce, and theferromagnetic crystalline grains exhibit magnetic anisotropyperpendicular to the film surface.
 5. A method of producing aperpendicular magnetic recording medium according to claim 4, whereinthe magnetic recording medium is deposited using a target composed of analloy and one of an oxide or a nitride, wherein the alloy is a CoPtalloy, an FePt alloy, or one of these alloys that contains additiveelement(s) selected from Cr, Ni, Nb, Ta, and B, and the oxide or thenitride is that of an element selected from Cr, Co, Si, Al, Ti, Ta, Hf,Zr, Y, and Ce.
 6. A method of producing a perpendicular magneticrecording medium according to claim 1, wherein the underlayer iscomposed of Ru or a Ru alloy.
 7. A method of producing a perpendicularmagnetic recording medium according to claim 6, wherein the Ru alloy isRuW, RuCu, RuC, RuB, or RuCoCr.
 8. A method of producing a perpendicularmagnetic recording medium according to claim 4, wherein the underlayeris composed of Ru or a Ru alloy.
 9. A method of producing aperpendicular magnetic recording medium according to claim 8, whereinthe Ru alloy is RuW, RuCu, RuC, RuB, or RuCoCr.
 10. A perpendicularmagnetic recording medium produced by the method according to claim 1.11. A perpendicular magnetic recording medium produced by the methodaccording to claim
 2. 12. A perpendicular magnetic recording mediumproduced by the method according to claim
 3. 13. A perpendicularmagnetic recording medium produced by the method according to claim 4.14. A perpendicular magnetic recording medium produced by the methodaccording to claim
 5. 15. A perpendicular magnetic recording mediumproduced by the method according to claim
 6. 16. A perpendicularmagnetic recording medium produced by the method according to claim 7.17. A perpendicular magnetic recording medium produced by the methodaccording to claim
 8. 18. A perpendicular magnetic recording mediumproduced by the method according to claim 9.